1. Field of the Invention
The present invention relates generally to printed circuit boards (PCBs) and, in particular, to a PCB that does not alter the characteristics of a radio frequency (RF) band pass filter (BPF) mounted thereon, and a method for making the same.
2. Description of the Related Art
In general, all the elements of a radio communications system are mounted on a PCB. For example, an RF BPF is separately procured and mounted on the PCB.
FIGS. 1A and 1B are top and bottom perspective views of an RF BPF, respectively, to which the present invention is applied. Here, the RF BPF is a dielectric filter, which is mounted on a PCB as shown in FIG. 2 by soldering. That is, an input terminal (IN) 10, an output terminal (OUT) 20, and three terminals 31, 32, and 33 of a ground terminal (GND) 30 of the BPF are mounted on an IN 40, and OUT 50, and three terminal 61, 62 and 63 of a GND 60 of the PCB, respectively.
When the dielectric RF BPF shown in FIGS. 1A and 1B is mounted on the PCB of FIG. 2, capacitance components exist between the IN 10 and the GND 30, and between the OUT 20 and the GND 30 due to the dielectric constant .epsilon. of the PCB. As a result, the characteristics of the BPF are altered. Thus, despite designing the BPF to satisfy characteristic requirements for an intended application, the BPF cannot reliably operate when mounted on the PCB (due to e.g., loss increase and changed center frequency).
FIGS. 3 and 4 are Smith charts illustrating the characteristics of the RF BPF when unmounted and mounted, respectively. The BPF characteristics shown in FIG. 3 meet the intended application requirements. However, the characteristics of the BPF become unsatisfactory when the BPF is mounted on the PCB. In the former case (FIG. 3), the impedance of the BPF is at an inductance location of the chart as indicated by wave "A", and the frequency bandwidth is wide as indicated by wave "B". On the other hand, in the latter case (FIG. 4), the impedance of the BPF includes more capacitance components as indicated by wave "A", and the frequency bandwidth becomes narrow as indicated by wave "B". Thus, while it is desirable for the RF BPF to exhibit the characteristics shown in FIG. 3 when the BPF is mounted on the PCB, the characteristics nonetheless change to those shown in FIG. 4. The changes to the characteristics arc attributable to the dielectric constant of the PCB material as described above. While the changes to the characteristics of the BPF can be prevented by re-designing the BPF, a redesign imposes serious constraints on a designer.